Chubu Electric Power Company has been implementing countermeasures in Hamaoka Nuclear Power Station (NPS) against tsunami and severe accidents following the disaster at Tokyo Electric Power Company's Fukushima Daiichi NPS that was caused by the 2011 Tohoku Earthquake. For this purpose, the L-shaped Tsunami protection wall 22 m high above sea level was constructed along coastline around the site. The total length of the protection wall was approximately 1.6 km and it was fixed to reinforced concrete underground walls, which were embedded in rock mass. Foundation rock consists of intercalated mudstone and sandstone. The tsunami protection wall has to withstand anticipated huge tsunamis such as the Nankai Trough Giant Tsunami, furthermore it has to withstand against the strong ground shaking of anticipated mega earthquakes to occur before the tsunami. The design seismic force is evaluated by the dynamic response analysis that can consider an interaction between structure, ground and bedrock, whereas the design wave force is set as hydrostatic pressure. Therefore, the tsunami protection wall must be designed reasonably based on the difference of seismic force and tsunami wave force. In this paper, the authors describe the concept of structural design on the basis of the above conditions and consider support performance of the protection wall and bedrock against the anticipated mega earthquake and huge tsunami.

1 Introduction
1.1 Overview of Hamaoka NPS

Chubu Electric Power supplies electric power to the central part of the main island of Japan facing the Pacific Ocean. The Hamaoka NPS is located in Shizuoka prefecture, and along the Pacific coast (Figure 1).

The Hamaoka NPS is Chubu Electric Power's only nuclear power station which has five nuclear power plants. Unit 1 and 2 are under decommissioning since 2009, and other 3 units are now waiting to restart. The total output of the remaining Units, 3, 4, and 5, is 3,617 MW.

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